fix v3 schema validation

[colearendt]

Close #11

This pulls the v3 schema verbatim from the google website. Unfortunately, the file's ordering and such has changed a decent bit. I changed a few things around, but ultimately thought sticking to a verbatim copy might be best.

I see this as fairly important since the plugin cannot be used with a content_security_policy until this fix is completed, since this causes a build failure, but satisfying the build causes a runtime failure.

# from the directory with the manifest schema
curl -sL -o schema-web-ext-manifest-v3.json https://json.schemastore.org/chrome-manifest